In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process coupled to the control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system used in the measurement, control, and monitoring of industrial processes. Typically, field devices are characterized by their ability to operate outdoors for extended periods of time, such as years. Thus, a field device is able to operate in a variety of climatological extremes, including severe temperature extremes and extremes in humidity. Moreover, field devices are able to function in the presence of significant vibration, such as vibration from adjacent machinery. Further, field devices may also operate in the presence of electromagnetic interference.
One example of a field device is a multivariable process fluid flow device, such as that sold under the trade designation Model 3051 SMV Multivariable Transmitter by Emerson Process Management of Chanhassen, Minn. Multivariable process fluid flow devices can compute mass flow rate through differential producers for liquids and gases. For some flow applications, it is desirable to know the energy flow rate in addition to, or instead of, the mass flow rate. In particular, these needs arise in the areas of natural gas flow and steam or water flow. For natural gas, the energy content, or heating value, of the fluid is completely specified by the gas composition and represents the amount of energy available if the natural gas were completely combusted. The desired units for such applications are typically Btu/unit time or Megajoules/unit time. For steam or water flow (useful in energy balance calculations) the energy content, or enthalpy, of the fluid represents the amount of energy required to bring steam or water to the pressure and temperature conditions of the flow. The enthalpy is usually referenced to a specific set of pressure and temperature conditions such as the melting point of water at atmospheric conditions. The desired units for such applications are also typically Btu/unit time or Megajoules/unit time.